Manufacture of seamless tubes made of steel or the like



June 9, 1964 H. BEN'n-:LER 3,136,185

MANUFACTURE OF SEAMLEISS TUBES MADE OF STEEL OR THE LIKE Original Filed May 15, 1956 '7 Sheets-Sheet 1 inventan '7 Sheets-Sheet 2 m am@ Inventor.' Helm 1+ en'hle Y H. BNTELER MANUFACTURE OF SEAMLEISS TUBES MADE OF STEEL OR THE LIKE Original Filed May 15, 1956 9T Ws. SCW@ June 9, 1964 H. BEN'n-:LER 3,136,135

MANUFACTURE OF' SEAMLESS TUBES MADE OF STEEL OR THE LIKE Original Filed May 15, 1956 7 Sheets-Sheet 3 fa I,

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Il Il l I I im June 9, 1964 H. BENTELER 3,135,185

MANUFACTURE 0F SEAMLESS TUBES MADE 0F STEEL OR THE LIKE Original Filed May l5, 1956 '7 Sheets-Sheet 4 June 9, 1964 H. BENTELER 3,136,185

MANUFACTURE OF SEAMLESS TUBES MADE OF STEEL OR THE LIKE Original Filed May 15, 1956 '7 Sheets-Sheet 5 la /S 7.9 ll'/ 50 2lb 17a Hdmu (Bc Mem- BtSf MM2 s. S45/#W June 9, 1964 H. EEEEEE ER 3,136,185

Original Filed May 15, 1956 Inventor: HWL* 'Budda r 95* WS Sm @dammi June 9, 1964 H. BEN-rl-:LER 3,135,185

MANUFACTURE 0F SEAMLESS TUBES MADE 0F STEEL 0R THE LIKE Original Filed May l5, 1956 7 Sheets-Shea?l '7 Inventar? Hdmd Bevdev B3: 'We mmf/LE ma q United States Patent O 3,136,185 MANUFACTURE F SEAMLESS TUBES MADE 0F STEEL 0R THE LIKE Helmut Benteler, Bielefeld, Germany, assigner, by mesue assignments, to Benteler-Werlre Aktiengesellschaft Werk Neuhaus, Schloss Neuhaus, Kreis Paderborn, Germany Continuation of application Ser. No. 535,036, May 15, 1956. This application Sept. 20, 1960, Ser. No. 57,290 Claims priority, application Germany May 17, 1955 3 Claims. (Ci. Sti- 62) The present invention relates to the manufacture of seamless metallic tubes made of such materials as steel and the like.

This application is a continuation application of the application Serial No. 585,036 led May 15, i956 and now abandoned.

In the manufacture of seamless tubes made of steel or the like, it is customary to hot roll a tube while reducing its cross section and simultaneously increasing its length until the desired cross section of the finished tube is obtained. According to this conventional process, it is not possible to obtain tubes with accurate inner dimensions, so that the tubes manufactured according to such a conventional process can only be used for simple purposes such as for conducting gas or water. Furthermore, the extent to which a tube may be reduced in cross section with the known tube manufacturing processes is limited. Thus, where a mandrel is used it is only possible to obtain an inner diameter of 50 mm. and Without a mandrel it is possible to provide with the conventional processes an inner diameter of approximately mm. To produce tubes of smaller cross sections and to produce tubes with accurate dimensions it is necessary to follow the hot rolling process with a cold drawing process and such a cold drawing is also required when the wall thickness of the tube is smaller than approximately 2.5 mm., because hot rolling of tubes with small Wall thickness is not possible. The cold drawing of such tubes is extremely inconvenient and expensive because of the many operations required in connection with cold drawing. Thus, it is necessary to divide the hot rolled tube up into convenient lengths before it can be cold drawn in order to further reduce its cross section, and between the individual passes of the cold drawing process it is necessary to anneal the tube in order to make it less brittle, this brittleness resulting from each cold drawing pass. This cold drawing is of further disadvantage because of the waste of material resulting therefrom as well as because of the relatively small speed of the tube during the cold drawing thereof, this speed being only approximately lttl-S0 meters per minute. Y

It is an object of the present invention to provide an apparatus and process capable of rolling a seamless tube of steel or the like in a manner much simpler than the conventional processes while retaining the accuracy of dimensions which could only be derived from the abovey described conventional cold ydrawing and in a manner which is less expensive than the conventional processes.

Another object of the present invention is to provide a tube rolling process and apparatus which is capable of reducing and elongating a tube while maintaining the tube in cold condition at all times and while handling the tube in a fully continuous manner.

Also, it is an object of the present invention to continuously roll a tube at a speed far greater than has heretofore been possible with conventional apparatus and processes so that the output of the apparatus and process of the present invention is far greater than conventional apparatus and processes.

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With the above objects in view the present invention mainly consists of a tube rolling process which includes the step of passing a tube in cold condition through a series of sets of compression rolls each of which engages the tube While it is located at any one of the sets along a plurality of spaced arcuate portions which have a total length of at least about half the periphery vof the tube. Furthermore, between each pair of successive sets of compression rolls there is provided by the difference in the peripheral speeds of the rolls a stretch ou the tube which is greater than that required to compensate for the increase in the length of the tube between each pair of successive sets of compressing rolls. Also, with the above objects in view the present vinvention mainly consists of a tube rolling apparatus which includes a plurality of rolls constituting a set of compression rolls, each of these rolls being formed with an annular groove having a central arcuate portion forming part of a circle and located nearer to the axis of the roll than lateral portions of the groove which respectively extend frorn the ends of the central portion thereof. These lateral groove portions are concave, extend up to the outer surface of the roll, and merge smoothly into the central portion of the groove. The plurality of rolls of each set are supported for rotation about their axes with these axes located in a common plane and with the rolls engaging each other at their outer surface portions which are located beyond the grooves formed therein, these grooves of each set of compression rolls cooperating with each other to form a passage having spaced arcuate portions, formed by the central portions of the groove, which form part of the same circle and havingy between these arcuate portions radial passage portions which extend radially beyondthe arcuate portions.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, in which:

FIG. l is a partly schematic, longitudinal sectional elevational View of a tube rolling mill according tothe present invention;

FIG. 2 is a partly sectional elevational view on an enlarged scale, as compared to FIG. 1, taken along line II-II of FIG. l in the direction of arrows;

FIG. 2a is a plan view on an enlarged scale of the transmission between members shown in FIG. 2,r part of the structure of FIG. 2a being broken away to clearly illustrate the important details vof the structure illustrated in FIG. 2;

FIG. 3 is a schematic illustration of the arrangement of the successive sets of rolls accordingto the present invention;

of FIG. 5 taken along line Vl-VI of FIG. 5 in the direcy tion of the arrows;

FIG. 7 is a transverse sectional View on an enlarged scale of a mandrel and tube as well as a pair of compressing rolls working on the tube;

FIG. 8 is a fragmentary, transverse, sectional View of a mandrel and tube and another embodiment of a set of compressing rolls acting on the tube;

FIG. 9 is a fragmentary sectional view taken along line IX--IX of FIG. 1 in the direction of the arrows and showing a mandrel and tube and only the parts of the compression rolls which act on the tube;

FIG. 10 is a View similar to FIG. 9 taken along line X-X of FIG. 1 in the direction of the arrows;

FIG. 11 is a view similar to FIG. 9 taken along line XI-XI of FIG. l in the direction of the arrows; FIG. 12 is a view similar to FIG. 9 taken along line XIIXII of FIG. l in the direction of the arrows;

FIG. 13 is a View similar to FIG. 9 taken along line XIII-XIII of FIG. 1 in the direction of the arrows; and

FIG. 14 is a view similar to FIG. 9 taken along line XIV- XIV of FIG. 1 in the direction of the arrows.

Referring to FIG. l, the elongating mill shown therein is composed of a plurality of roll assemblies A, B, C, D, and each of these assemblies includes two sets of compressing rolls and one set of rounding rolls. Thus, the sets of rolls a, b, d, e, g, h, k, and l are sets of compressing rolls, while the sets of rolls c, f, i, m are sets of rounding rolls. Thus, each of the above assemblies which are arranged one behind the other are composed of a pair of sets of compressing rolls located one behind the other and followed by a set of rounding rolls. For the sake of simplicity, the set of rolls which are illustrated in FIG. 1 are shown as if they all included rolls turnable about axes normal to the plane in which FIG. 1 is taken. Actually, this is not the case, since each set of rolls is angularly displaced with respect to the preceding and the following set of rolls, as is apparent from FIG. 3 where the succeeding set of rolls are schematically illustrated.

As may be seen from FIG. 2, each set of compressing rolls 2 is driven by an electric motor 1. The motor I may be an adjustable rotating field induction motor. Preferably a separate motor is provided to drive each set of compression rollers, while each set of rounding rolls may be driven from the same motor which drives the preceding set of compression rolls. The motor 1 drives the compression rolls 2 through a clutch 3 which transmits the drive to a bevel gear drive 4 which in turn transmits the drive to a telescoped shaft arangement S. The upper end of this telescoped shaft arrangement 5, as viewed in FIG. 2, is connected to a second bevel gear drive which cooperates directly with a reduction gear unit 8, as is shown on an enlarged scale in FIG. 2a. The reduction gear assembly 8 operates a further pair of gears 9a and 9b, and these gears are connected through universal joints with shafts 10a and 10b, respectively, which are in turn connected through universal joints with the compression rolls 11a and 11b. As is evident from FIG. 2a, the pair of bevel gears 6 drive the pair of bevel gears 7 which in turn transmit the drive to the reduction gear assembly 8.

As may be seen from FIGS. 5 and 6, the pair of compression rolls 11a and 11b have a relatively small diameter and are in engagement with a pair of larger rolls provided for each of the compression rolls. Thus, the compression roll 11b engages at its side directed away from the roll 11a a pair of larger rolls 13a and 13b, while the compression roll 11a engages at its side ldirected away from the compression roll 11b the pair of larger rolls IZa and 12b. The free ends of these rolls 12a, 12b, 13a, 13b are turnable in bearings which form part of housings 17a and 17b, the housings 17a being shown at the left of FIG. 5 connected to the .left ends of the rolls 12a-13b while the housings 17b at the right side of FIG. 5 turnably support the ends of the rolls 12a-13b which are located to the right, as viewed in FIG. 5. These housings 17a and 17b are threadedly connected to elongated spindles 18a and 18b which are in turn Xed to gears so that they maybe turned, the portions of the spindles 18a and 18h which engage the bearing housings being threaded oppositely in such a way that the bearing housings connected to the rolls 13a and 13b always move in equal and opposite directions with respect to the bearing housings connected to the rolls 12a and 12b. By regulating the distance between the rolls 12a and 12b on the one hand and the rolls 13a and 13b on the other hand it is possible to regulate the distance between the working rolls 11a and 1lb. With a structure which is not shown in the drawing the rolls IIa and 11b are maintained against lateral shifting.

As may be seen from FIG. 4, each pair of rounding rolls 16a and 16h have their free ends turnably supported in additional housings 17a and 1'7b which cooperate in the same way with spindles 18a and 18b. In order to adjust the spindles 18a and 18h they are fixed to gears I9 and 19a which serve to turn the spindles. A hand wheel 2t) is fixed to the gear 19a so that when the hand wheel 20 is turned the housings 17a and 17b will shift so as to locate the rolls 16a and 1Gb at a desired distance from each other in the case of FIG. 4 and so as to locate the rolls 11a and 11b at a desired distance from each other in the case of FIGS. 5 and 6. The structure of FIGS. 4-6 is identical for all of the sets of compression rolls and rounding rolls. As is evident from FIG. 4, the rounding rolls are respectively driven by a pair of shafts 21a and 2lb connected to the rolls through universal joints, and these shafts 21a and 2lb are connected through additional universal joints as well as bevel gear and reduction gear assemblies to the motor of the compression roll set which is located just preceding the particular rounding roll set.

Instead of providing a mill which includes four assemblies of rolls each including two sets of compression rolls and one set of rounding rolls, it is possible to provide a smaller or greater number of roll assemblies. Also, when the tube is cold rolled without a mandrel, it is possible to partially or completely eliminate the rounding rolls.

As may be seen from FIG. 3, the successive sets of rolls are angularly displaced with respect to each other. FIG. 3 schematically illustrates the arrangement of compression and rounding rolls when the cold working takes place on a cylindrical mandrel having a uniform diameter along its entire length. The Vangular displacement of the several sets of rolls with respect to each other is diagrammatically shown in FIG. 3a. FIG. 3a shows the angular positions of the sets of rolls with respect to each other where the rst set of compression rolls a is indicated with a vertical line, as shown in FIG. 3a. The rolls of the first assembly A have their angular positions indicated in the outermost circle of FIG. 3a, while the angular positions of the succeeding assemblies B, C, D are shown in FIG. 3 in concentric circles respectively located within each other and within the outermost circle, as is evident from FIG. 3a. As may be seen from FIGS. 3 and 3a, the pair of compression rolls of the second set of compression rolls of each assembly is displaced by with respect to the first set of compression rolls of each assembly. The set of rounding rolls of each assembly is displaced by 45 with respect to the compression rolls. The sets of compression rolls of one assembly are angularly displaced with thesets of compression rolls of the next assembly by either 22.5 or 45". The angular positions of the sets of rolls of the first two assemblies A and B are further illustrated in FIGS. 9-14.

The successive sets of rolls are operated at peripheral speeds greater than that of the next preceding set of rolls so as to compensate for the elongation of the tube due to the reduction in cross-section and wall-thickness and to the advance of the tube at each set of rolls. Furthermore, in accordance with the present invention, the successive sets of rolls are operated at a peripheral speed even higher than this peripheral speed. Thus, the peripheral speed of the second set of compression rolls of` each assembly is higher than that of the first set of compression rolls of each assembly by an amount greater than that required to compensate for the increase in length of the tube so that the pairs of compression rolls of each assembly provide a positive tension and stretch in the portion of the tube which is located between each pair of compression roll sets at any given instant. The increase in the peripheral speed of the second set of compression rolls of each assembly, depending upon the linear speed of the tube and the cross section thereof will be approximately 2-5% more than the speed of the second set of rolls which would be provided if a positive tension were not applied to the tube during passage thereof through the elongating mill of the invention.

As may he seen from FIG. 7, each pair of compression rolls 11a and 11b is provided with an annular groove. Each of the rolls 11a and 1lb has such a groove formed in its outer face, and this groove has in a diametral plane an inner central portion located nearer to the axis of the roll than the lateral portions of the groove which respectively extend beyond the ends of the central portion. As is evident from FIG. 7, these lateral groove portions are concave and merge smoothly into the central portion of the groove. Thus, the pair of rolls 11a and 11b cooperate with each other to forma passage having the configuration shown in FIG. 7, and as may be seen from FIG. 7 this passage has a pair of opposed arcuate portions forming part of the same circle and between these arcuate portions a pair of lateral passage portions which extend radially beyond the arcuate portions. As is apparent from FIG. 7, with this arrangement each pair of rolls 11a and 11b will engage the tube 14 along a pair of arcuate peripheral portions which form part of the same circle and which are spaced from each other in order to leave between these arcuate portions a pair of portions of the tube 14 which are not compressed by the rolls. It is preferred to make the arcuate portions of the tube which are compressed by the rolls of such a size that they totally extend at least about approximately half the periphery of the tube, which corresponds to a total angle of at least slightly more than 180. As may be seen from FIG. 7, the arcuate portions of the tube 14 which are engaged by the rolls 11a and 11b are pressed against the mandrel 15, and in the example shown in FIG. 7 the angle a through which each roll 11a compresses the tube 14 is approximately 110, so that in the example of FIG. 7 the tube 14 is compressed along a total peripheral angle of 220. The compression of the tube, as shown in FIG. 7, reduces the wall thickness of the tube at the part thereof engaged by the compression rolls 11a and 11b and at the same time the tube is stretched longitudinally. As is apparent from FIG. 7, the laterally extending passage portions are long enough to enable the elongated portions of the tube 14 which are not compressed to remain out of engagement with the rolls 11a and 11b as Well as out of engagement with the mandrel 15. By the elongation of the tube 14 at the portions thereof which are compressed by the rolls 11a and 11b a tensile force is provided on the lateral portions of the tube 14 which are not compressed greater than the yield point of the material of the tube in these not compressed portions so that these portions of the tube 14 which are not compressed are nevertheless stretched and elongated during passage of the tube through the mill of the invention.

If the second set of compression rolls of each of the assemblies A-D had a peripheral speed greater than the rst set by an amount only sufficient to compensate for the increase in length of the tube between each pair of successive compression rolls, the elongation of the tube would still provide a tensile force at the portions'of the tube which are not compressed which would be capable of stretching these portions, but at the regions vof the tube portions which are not compressed and which are located next to the compressed portions of the tube there would be caused by the rolls 11a and 11b due to the reduction of the Wall thickness of the tube an increase in the only to an extremely small extent.

peripheral length of the tube. Thus, as was mentioned above, the second set of compression rolls of each assembly is driven at a peripheral speed slightly greater than that required to compensate for the increase in length of the tube resulting from the reduction in cross section thereof, and this increased increment of peripheral speed positively stretches the tube at the portions thereof next to the compressed portions of the tube, and the stretching of the tube at these portions is suicient to cause the material of the tube to be displaced only longitudinally of the tube and an increase in the peripheral length of the tube at a part thereof engaged by the compression rolls at any given instant is avoided in this way. Furthermore, the increased speed of the second set of compression rolls of each assembly augments the longitudinal stretching of the uncompressed parts of the' tube and reduces the resistance of the tube to deformationrat the regions of the tube which are not actually compressed by the rolls.

FIG. 8 illustrates an embodiment of the invention where each set of compression rolls includes three rolls 22a., 22b, 22C which have turning axes located in a common plane and displaced from each other by These rolls are also formed with annular grooves of substantially the same conguration as the grooves of the rolls 11a and 11b although they are of a smaller size. Thus, each of the grooves of the rolls 22a-22e has a central arcuate portion forming part of a circle and extending in the example shown in FIG. 8 through an angle of approximately 70. Thus, the tube 14 in the embodiment of FIG. 8 will be engaged along three spaced arcuate portions forming part of the same circle and having a total arcuate length of 210. With the embodiment of FIG. 8, as with the embodiment of FIG. 7, the grooves of the several rolls form radially extending passage portions which extend beyond the arcuate portions of the grooves which form part of the same circle and which provide spaces in which portions of the tube 14 may be located without being compressed, these portions of the tube 14 being, however, longitudinally stretched in the manner described above.

With the apparatus and process of the invention, the portions of the tube which are actually engaged and compres-sed by the compression rolls have a tensile strength which is substantially greater than that of the uncompressed portions of the tube which are not engaged by the compression rolls. Because of the smaller tensile strength of these uncompressed elongated portions of the tube, the tensile force which is provided by the sets of compression rolls is great enough to exceed the yield point of the uncompressed portions of the tube so as to provide a stretch and an elongation of these uncompressed portions of the tube. In order to stretch the uncompressed portions of the tube it is only necessary to exceed the yielding point of the material in the uncompressed portions of the tube, which is l0 to 20% less than the tensile strength in the compressed portions of the tube. By engaging the tube with compression rolls which engage the tube at spaced arcuate portions having a total peripheral length of at least approximately half the periphery of the tube, the material of the tube is longitudinally rolled and the spreading of the material of the tube peripherally about the axis of the tube takes place By providing the above mentioned peripheral speed of the second set of compression rolls of each assembly which is greater by a predetermined amount than the peripheral speed necessary to compensate for the increase in length of the tube due to its compression, a positive stretch of the tube in the regions thereof next to the compressed portions thereof is provided and this positive stretching also works on the outermost portions of the tube which are not compressed.

In this way, in spite of the relatively large reduction in the cross section of the tube during passage thereof through the elongating mill of the invention there is only a small stiifening of the tube with an extremely small increase in the brittleness of the tube. In other words, with the process of the invention the ductility of the material of the tube is maintained in spite of the relatively large reduction in cross section of the tube during one pass through the mill of the invention. Because of the extremely strong longitudinal stretching of the tube during the cold working thereof with the process and apparatus of the invention an extremely ne grain structure is provided, and this is a grain structure which is superior to that of the material of the tube supplied to the apparatus of the invention before this tube is worked on according to the process and apparatus of the invention. This ne grain structure is obtained when the tube is annealed after passing through the rolls, as is conventional after cold rolling processes.

While the tube is being worked according to the process and apparatus of the present invention, the temperature of the tube is maintained lower than 150 C. and preferably lower than 100 C. The major part of the heat generated in the tube during the Working thereof is carried Vaway by the rolls which engage the tube as Well as by the mandrel. If insucient heat is carried away by the rolls and the mandrel, then the tube may be cooled with a positive cooling means during the rolling thereof. For this purpose the rolls may be cooled in their interior portions in any suitable manner or the exterior of the tube may have a cooling liquid sprayed thereon, or both of these expedients may be resorted to simultaneously, Where necessary.

The maintenance of this relatively low temperature of the tube is of importance since below these temperatures the resistance of the tube where it is made of steel to deformation is at a minimum while at the higher temperatures the resistance to deformation increases and only starts to decrease again at a temperature of more than approximately 300 C. where the tube is made of steel.

The following is an example of an actual process carried out with the apparatus of the invention. The tube placed in the mill of the invention was made of steel and had a tensile strength of 35 kg./mm.2, and this tube had an elongation of approximately 30-3570. This tube had an exterior diameter of 25 mm. and a wall thickness of 2.5 mm. The tube was rolled in the four assemblies, A, B, C, D while using a mandrel located within the tube and moving with the same through the rolls. This mandrel together With the rolls resulted in a nished tube whose outer diameter was mm. and whose wall thickness was 0.8 mm. The reduction in cross section of the tube was distributed among the several assemblies as follows:

Within the assembly A the reduction in cross section was 33%, the first set of compression rolls a providing a cross section reduction of 19% and the second set of compression rolls b providing a cross section reduction of 18.2%. In the second assembly B the reduction in cross section of the tube was 30.2%, and the rst set of compression rolls d provided a cross section reduction of 17.3% while the second set of compression rolls e provided a cross section reduction of 15.6%. In the third assembly C the cross section of the tube was reduced 27.3%, and the rst set of compression rolls g provided a reduction of 15.4% while the second set of compression rolls h provided a reduction of 14.4%. In the last assembly D the cross section of the tube was reduced 25.8%, and the rst set of compression rolls k provided a reduction of 13.9% while the second set of compression rolls l provided a reduction in cross section of 12.7%.

These percentages relate to the total cross sectional area;

of the tube between the time when it entered a particular set of compression rolls and the time when it left the same set of compression rolls or, in the case of the reduction of cross section of an assembly of compression rolls the total cross sectional area of the tube when it entered the assembly and when it left the assembly. The entire reduction in the cross section produced by the entire mill was approximately 74%. The finished tube after leaving the mill had a tensile strength of 65 kg./mm.2 and an elongation of approximately 6-8%.

Where necessary, the finished tube can be again rolled in a similar manner with similar apparatus without the mandrel, if desired, so as to have its cross section reduced still further. In this way it is possible to obtain tubes having an inner diameter of approximately 4 mm. and smaller.

With the process of the invention it is possible to provide accurate steel tubes in cold condition of such small dimensions as could only be obtained heretofore by cold drawing of the tubes after hot rolling thereof. The process of the invention is particularly suitable for the manufacture of tubes where the tube at the beginning has an outer diameter of less than approximately mm. To manufacture finished tubes with an inner diameter of more than 10 mm., the tube which is worked on is preferably provided in its interior with a mandrel of uniform diameter and cylindrical cross section which moves with the tube through the mill. During such rolling of the tube the wall thickness thereof decreases. When the inner diameter of the nished tube is less than approximately 10 mm., the mandrel is not used, and in this way it is possible to provide tubes having an inner diameter of less than 4 mm. and a wall thickness of less than l mm. In contrast to the conventional cold drawing of tubes, a great advantage of the process and apparatus of the invention resides in the speed of movement of the tube with the process and apparatus of the present invention. Thus, it is possible to provide in one pass a cross section reduction of a tube of more than 70% while the tube moves at a linear speed of more than meters per minute. Also, it is possible to provide further reductions in cross section by moving the tube through several passes through additional mills which further reduce the cross section of the tube, and between such a plurality of passes the tube may be annealed.

It will be understood that each of the elements described above, or two or more together, may also nd a useful application in other types of tube rolling process and apparatus differing from the types described above.

While the invention has been illustrated and described as embodied in tube elongating process and apparatus, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from 'the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. A process for cold rolling of a small diameter steel tube comprising the steps of passing a steel tube having an inner diameter of between 10 and 20 mm. in cold condition with an elongated cylindrical mandrel bar of uniform cross section and a diameter substantially smaller than the inner diameter of the tube located in the interior of said tube along a plurality of successive rolling stations, and rolling said tube at each of said rolling stations along a plurality of spaced arcuate portions distributed about the axis of the tube and forming part of a circle concentric to said axis and having a total peripheral length about said axis greater than one half of the periphery of the tube so as to compress said spaced arcuate portions against said mandrel while simultane-` ously leaving between said thus compressed rolled arcuate portions a plurality of irl-between tube portions which are not subjected to rolling and thus are uncompressed and which are stressed in longitudinal direction of the tube by the elongation of the tube due to said arcuate stations thereof being rolled, said spaced arcuate portions being rolled at any one of said rolling stations being angularly displaced with respect to the spaced arcuate portions being rolled at the preceding and at the following rolling station, said rolling at any pair of consecutive rolling stations being carried out at a speed difference greater than that required to compensate for elongation of the tube between the respective pair of consecutive rolling stations so as to provide a positive stretch at least at those portions of the tube which are located adjacent said arcuate portions thereof being rolled and so as to maintain all portions of the steel tube during cold rolling in cold flow condition.

2. A process for cold rolling of a small diameter steel tube as defined in claim 1 and comprising the additional step of maintaining the steel tube during rolling at all times at a temperature below 150 C.

3. A process for cold rolling of a small diameter steel tube as deiined in claim 1 in which said tube is rolled at a rolling speed of over 120 meters per minute and in which said positive stretch is great enough to subject said iii-between tube portions to tension forces exceeding the yield point of the cold tube material.

4. A process for cold rolling of a small diameter steel tube as dened in claim 1 in which the positive stretch imparted to said in-between tube portions is great enough to limit radial spread of said portions so as to maintain said portions out of engagement with peripheral portions of the rolls in each of said rolling stations.

5. A process for cold rolling of a small diameter steel tube as defined in claim 1 in which the maximum total peripheral length of said arcuate portions is about 220.

6. A process for cold rolling of a small diameter steel tube comprising the steps of passing a steel tube having an inner diameter of less than mm. in cold condition and with the interior of the tube left empty along a plurality of successive rolling stations, and rolling said tube at each of said rolling stations along a plurality of spaced arcuate portions distributed about the axis of the tube and forming part of a circle concentric to said axis and having a total peripheral length about said axis greater than one half of the periphery of the tube so as to compress said spaced arcuate portions While simultaneously leaving between said thus compressed rolled arcuate portions a plurality of in-between tube portions which are not subjected to rolling and thus are uncompressed and which are stressed in longitudinal direction of the tube by the elongation of the tube due to said arcuate stations thereof being rolled, said spaced arcuate portions being rolled at any one of said rolling stations being angularly displaced with respect to the spaced arcuate portions being rolled at the preceding and at the following rolling station, said rolling at any pair of consecutive rolling stations being carried out at a speed difference greater than that required to compensate for elongation of the tube between the respective pair of consecutive rolling stations so as to provide a positive stretch at least at those portions of the tube which are located adjacent said arcuate portions thereof being rolled and so as to maintain all portions of the steel tube during cold rolling in cold tlow condition.

7. A process for cold rolling of a small diameter steel tube as set forth in claim 6 and comprising the additional step of maintaining the steel tube during rolling at all times at a temperature below C.

8. A process for cold rolling of a small diameter steel tube as defined in claim 6 in which the maximum total peripheral length of said arcuate portions is about 220.

References Cited in the file of this patent UNITED STATES PATENTS 1,858,990 Foren May 17, 1932 1,888,607 Otfutt Nov. 22, 1932 1,926,237 Inslee Sept. 12, 1933 1,934,844 Diescher Nov. 14, 1933 2,041,937 Korbuly May 26, 1936 2,085,968 Gassen July 6, 1937 OTHER REFERENCES The Rolling Mill Journa, volume 7, No. 2, page 98-106. The Fundamentals of Continuous Tube Rolling, G. B. Lobkowitz. 

1. A PROCESS FOR COLD ROLLING OF A SMALL DIAMETER STEEL TUBE COMPRISING THE STEPS OF PASSING A STEEL TUBE HAVING AN INNER DIAMETER OF BETWEEN 10 AND 20 MM. IN COLD CONDITION WITH AN ELONGATED CYLINDRICAL MANDREL BAR OF UNIFORM CROSS SECTION AND A DIAMETER SUBSTANTIALLY SMALLER THAN THE INNER DIAMETER OF THE TUBE LOCATED IN THE INTERIOR OF SAID TUBE ALONG A PLURALITY OF SUCCESSIVE ROLLING STATIONS, AND ROLLING SAID TUBE AT EACH OF SAID ROLLING STATIONS ALONG A PLURALITY OF SPACED ARCUATE PORTIONS DISTRIBUTED ABOUT THE AXIS OF THE TUBE AND FORMING PART OF A CIRCLE CONCENTRIC TO SAID AXIS AND HAVING A TOTAL PERIPHERAL LENGTH ABOUT SAID AXIS GREATER THAN ONE HALF OF THE PERIPHERY OF THE TUBE SO AS TO COMPRESS SAID SPACED ARCUATE PORTIONS AGAINST SAID MANDREL WHILE SIMULTANEOUSLY LEAVING BETWEEN SAID THUS COMPRESSED ROLLED ARCUATE PORTIONS A PLURALITY OF IN-BETWEEN TUBE PORTIONS WHICH ARE NOT SUBJECTED TO ROLLING AND THUS ARE UNCOMPRESSED AND WHICH ARE STRESSED IN LONGITUDINAL DIRECTION OF THE TUBE BY THE ELONGATION OF THE TUBE DUE TO SAID ARCUATE STATIONS THEREOF BEING ROLLED, SAID SPACED ARCUATE PORTIONS BEING ROLLED AT ANY ONE OF SAID ROLLING STATIONS BEING ANGULARLY DISPLACED WITH RESPECT TO THE SPACED ARCUATE PORTIONS BEING ROLLED AT THE PRECEDING AND AT THE FOLLOWING ROLLING STATION, SAID ROLLING AT ANY PAIR OF CONSECUTIVE ROLLING STATIONS BEING CARRIED OUT AT A SPEED DIFFERENCE GREATER THAN THAT REQUIRED TO COMPENSATE FOR ELONGATION OF THE TUBE BETWEEN THE RESPECTIVE PAIR OF CONSECUTIVE ROLLING STATIONS SO AS TO PROVIDE A POSITIVE STRETCH AT LEAST AT THOSE PORTIONS OF THE TUBE WHICH ARE LOCATED ADJACENT SAID ARCUATE PORTIONS THEREOF BEING ROLLED AND SO AS TO MAINTAIN ALL PORTIONS OF THE STEEL TUBE DURING COLD ROLLING IN COLD FLOW CONDITION. 